1. Field of the Invention
This invention relates to ink jet technology, and more particularly to method and apparatus for controlling the trajectory of a continuous stream of ink droplets in their path to a recording medium.
2. Prior Art
In one form of ink jet printing, conductive fluid is delivered under pressure from a cavity through an orifice in the form of a continuous stream. Perturbation is applied to the ink in the cavity, such as for example, by periodic excitation of a piezoelectric crystal mounted within the cavity. This excitation causes the continuous stream flowing through the orifice to break up into substantially uniform drops which are uniformly spaced from one another. At the point of drop formation, drop charge electrodes coupled to control circuitry for applying specific voltages induce a charge upon the drops. Selective deflection of the drops is then achieved by passing them through an electric field created by deflection electrodes having a voltage sufficient to cause an appreciable drop deflection. The electric field generated by the electrodes selectively deflects the charged drop to a predetermined position on a recording medium or to a gutter which is coupled to the cavity and is utilized to recycle those ink droplets not directed to the recording medium.
A number of ink jet geometries have been proposed to encode information on a record medium such as a sheet of paper. In a typical ink jet configuration ink droplets are selectively transmitted to the sheet of paper a row at a time and the sheet is moved in relation to the ink jet generator so that subsequent rows may be encoded with information. The longitudinal movement between paper and ink jet generator may, for example, be achieved by mounting the paper to a rotating support drum which causes the paper to move past the generator.
According to one ink jet technique, a single jet nozzle sweeps or scans back and forth across the paper at a high rate of speed, depositing ink in both directions of the scan. A system embodying a single ink jet nozzle must include apparatus to accurately accelerate and decelerate that nozzle for each row of the scan. Use of a single ink jet nozzle places an upper limit on the speed with which the paper can be moved past the generator.
One proposed solution to the speed constraint imposed by the single ink jet geometry requires a 1:1 correspondence between the number of ink jet nozzles and the number of pixels or incremental areas of coverage across the width of paper. These multiple nozzles are stationary with respect to the paper and, therefore, require no controlled accelerations. A problem encountered with this ink jet geometry is the close spacing required to achieve a high resolution encoding of ink onto the paper. The ink jet charging and deflecting circuitry must also be closely spaced. This geometry becomes untenable for any system requiring high resolution.
The problems encountered with the single nozzle and 1:1 geometries discussed above have led to the proposal of an ink jet system having multiple ink jet nozzles which are spaced apart and thereby supply ink droplets to multiple pixels in a given scanning row. Choice of this intermediate geometry requires some mechanism or technique for providing complete coverage across a given row of pixels. One technique for providing this coverage is proposed in U.S. Pat. No. 3,689,693 to Cahill et al. entitled "Multiple Head Ink Drop Graphic Generator." Apparatus constructed in accordance with the '693 patent requires transverse or side to side scanning of the multiple ink jets so that each jet is responsible for sending ink droplets to a number of pixels in a given row. The vertical movement of the paper with respect to the ink jet nozzles may be intermittent or continuous. If the movement is intermittent, each ink jet sweeps across its entire segment of coverage before the paper is stepped to a new position. In a continuous motion system the paper is mounted to a rotating drum and each jet sweeps off a spiralling trajectory, moving sideways one pixel per drum revolution.
A somewhat different approach for a multiple jet spaced apart ink jet system is proposed in U.S. Pat. No. 4,274,100 to Stephen F. Pond entitled "Electrostatic Scanning Ink Jet Method and Apparatus" which was filed in the U.S. Pat. Office on Oct. 4, 1978. The Pond patent is incorporated herein by reference. The apparatus described in that patent includes a series of spaced multiple ink jets which provide complete scanning coverage across a given row of pixels on the record medium without requiring side to side movement of the multiple ink jet nozzles. Each ink jet has associated with it a number of charging and deflection elements which interact with an ink drop to control its trajectory. Of particular note is the utilization of a control electrode or electrodes which repetitively cause a given ink jet to scan in a horizontal direction across a portion of a width of the record medium. Use of multiple ink jets provides coverage for an entire row. This ink deflection is provided prior to the breakup into individual drops and once break up does occur the drops are charged to an appropriate level, so that a deflection electrode can be used to controllably direct those drops either to the record member or to a gutter.
The apparatus disclosed in the Pond patent represents a significant advance over the art. An entire row of pixels on the record member can be selectively encoded with information without moving the plurality of spaced ink jets in relation to the sheet of paper. Practice of the invention disclosed in the Pond patent is not achieved without a certain degree of complexity. Care must be taken in applying control voltages to the electrodes to ensure that each of the multiple ink jets cover its designated region across the width of paper without overlapping its next closest neighbor and also without leaving gaps between areas of coverage. The process ensuring complete coverage across the width of the sheet of paper is known in the art as stitching.
A second U.S. patent application entitled "Linear Ink Jet Deflection Method and Apparatus" to Torpey, U.S. Ser. No. 251,405 relates to an improved circuit for controlling the lateral deflection of the ink column in a Pond type ink jet apparatus. According to the technique disclosed in the Torpey apparatus, two electrodes spaced on opposite sides of an ink jet column deflect the column. By control of the voltages applied to these oppositely positioned electrodes, the angle of ink jet column deflection has been made proportional to a control voltage applied to the electrodes. This proportionality facilitates control over column scanning to insure proper stitching together of ink droplets from a plurality of ink jet nozzles in such a system.
The present invention relates to an improvement in the Pond type ink jet configuration which includes a focusing/defocusing electrode which deflects charged ink droplets subsequent to ink passage past the column deflection electrode. Use of an electrode positioned downstream from a point of droplet breakoff to focus charged droplets is not new. U.S. Pat. No. 4,224,523 to Crean, for example, shows a focusing "lens" which deflects or focuses charged droplets to a common fuel focal line on a recording medium. The Crean apparatus was not used in a Pond type system. It was used to compensate for misdirected jet columns and not for controlled scanning of droplets to the plane of the recording medium.